Kinetics of SonophotocatalyticDegradation of Anionic Dyes withNano-TiO2

R . V I N U A N D G I R I D H A R M A D R A S *

Department of Chemical Engineering Indian Institute ofScience, Bangalore, 560012, India

Received September 10, 2008. Revised manuscript receivedNovember 7, 2008. Accepted November 12, 2008.

The current research work focuses on the combination ofphotocatalyticandsonocatalytic (sonophotocatalytic)degradationof anionic dyes, viz., Orange G, Remazol Brilliant Blue R,Alizarin Red S, Methyl Blue, and Indigo Carmine, with solutioncombustion synthesized TiO2 (CS TiO2) and commercialDegussa P-25 TiO2 (DP-25). The rate of sonophotocatalyticdegradation of all the dyes and the reduction of total organiccarbon was higher compared to the individual photo- andsonocatalytic processes. The effect of dissolved gases andultrasonic intensity on the sonophotocatalytic degradation of thedyes was evaluated. A dual-pathway network mechanism ofsonophotocatalytic degradation was proposed for the first time,and the rate equations were modeled using the networkreduction technique. The kinetic rate coefficients of the individualsteps were evaluated for all the systems by fitting the modelwith experimental data.

IntroductionDyes constitute a major class of organic compounds, whichfind a multitude of applications in our daily life in paper,leather, cosmetics, plastics, clothing, drugs, electronics, andprinting. The modern textile industry consumes about 80%of the synthetic dyes produced, with 700 000 tons of dyesapplied to 40 million tons of fabric annually. Besidesconsuming large amounts of other chemicals for processing,dyeing also generates effluents, that contain 10-15% of thedye (1). Hence, reducing the toxicity levels to permissiblelimits before admitting the dye wastewater to aquatic bodiesis a critical issue.

Among the various treatment methods like neutralizationof acidic and alkaline effluents, flocculation, aerobic, andanaerobic biological treatments, advanced oxidation pro-cesses (2) like ultraviolet (UV) photolysis, UV/H2O2 process,UV/O3 process, UV/Fenton process, and photocatalyticprocess have gained considerable interest due to theireffective decontamination efficiencies. Among all the abovemethods, photocatalysis, with TiO2 as the semiconductorphotocatalyst has emerged as a promising technique owingto the high photoactivity, optimum band gap, and ease ofsynthesis of TiO2. Recently, we have synthesized anatasephase TiO2 by solution combustion methodology (3, 4), andits superior photoactivity for the degradation of dyes hasbeen extensively evaluated (3, 5). TiO2 photocatalysis pro-ceeds by the generation of charge-carriers, which producehydroxyl radicals, and the mechanism is well understood(6).

Recently, there has been an immense thrust in couplingultrasound (US) with UV irradiation to enhance the overalldegradation of the substrate. The ability of US to promotea chemical reaction can be attributed to acoustic cavitation(7-10). Cavitation involves the nucleation, growth/expansionand collapse of the microbubbles by implosion. Pockets ofdissolved gases in the solution act as first nucleation sitesand as the cavitation bubbles implode, they give rise to newmicrobubbles. During the growth phase, the oscillation ofthe microbubbles can either be stable or transient dependingon the intensity and frequency of US. The sudden collapseof the microbubbles result in localized high temperatures(g5000 K) and pressures (g1000 atm), which result in thethermal dissociation of water to form hydroxyl and hydrogenradicals, which are the precursors of any chemical reactioninduced by US. For the sonocatalytic process, only Priya andMadras (11) and Kubo et al. (12) have arrived at themechanistic kinetic model for the degradation of Rhodaminedyes and phenol, respectively.

In the recent past, many groups have studied thesonophotocatalytic degradation of dyes (13-16), specificallysulfonated azo dyes. The rate data has been modeled usingpseudo first order kinetics, without incorporating the mecha-nistic aspects of the UV and US processes. Hence, it is theaim of this study to develop a kinetic model for sonopho-tocatalytic degradation of dyes based on the underlyingmechanism. We have adopted the multipathway networkreduction technique (17) to arrive at the rate equation andevaluate the rate coefficients of the individual steps of thedegradation process. Previously, we have adopted dual-cyclenetwork reduction technique to model the simultaneousphotocatalytic degradation of phenolic compounds andreduction of metal ions (18). We have used anionic dyes,which are mostly used to dye nylon, silk, wool, and modifiedacrylics, as model substrates to evaluate the sonophoto-catalytic activity in the presence of combustion synthesizedTiO2 (CS TiO2) and commercially available Degussa P-25 TiO2

(DP-25). The effect of dissolved gases and US intensity onthe sonophotocatalytic degradation of the dyes has also beenevaluated. The dyes belong to four different classes based ontheir structure, viz., Orange G (OG, C16H10N2Na2S2O7, azoic),Remazol Brilliant Blue R (RBBR, C22H16N2Na2S3O11) andAlizarin Red S (ARS, C14H7NaSO7) (anthraquinonic), MethylBlue (MB, C37H27N3Na2S3O9, triphenyl methane), and IndigoCarmine (IC, C16H8N2Na2S2O8, indigo).

Experimental SectionCatalyst Preparation and Characterization. Solution com-bustion synthesis methodology (3, 4) was adopted for thesynthesis of nano TiO2. The synthesis procedure andcharacterization of the catalyst by different techniques aregiven in Supporting Information SI 1.

Sonophotocatalytic Reactor. The reactor consisted of ajacketed borosilicate glass container of 5.2 cm i.d., 7.6 cmo.d., and 10 cm height, in which the dye solution wasdegraded. A horn type sonicator (Vibronics, India) of 36 Wcm-2 average rated power (by the manufacturer) and 25 kHzfrequency, attached to a pair of lead titanate zirconatetransducer elements was used. The horn was dipped 2 cmabove the bottom of the container into the solution tofacilitate continuous stirring using a magnetic stirrer. UVlight source was 80 W high pressure mercury vapor lamp(HPML) placed inside a jacketed quartz tube of 4 cm i.d., 4.7cm o.d., and 18 cm height. The distance between the UVlight source and the reaction vessel was 1 cm, as shown inthe schematic diagram (see Figure SI 2 in the Supporting

* Corresponding author phone: +91-80-22932321; Fax: +91-80-23600683; e-mail: giridhar@chemeng.iisc.ernet.in.

Environ. Sci. Technol. 2009, 43, 473–479

10.1021/es8025648 CCC: $40.75 2009 American Chemical Society VOL. 43, NO. 2, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 473

Published on Web 12/19/2008

Information). Cold water was circulated in the jacket of thecontainer and in the annulus of the quartz tube to quenchthe heat generated and to maintain the reaction temperaturewithin 35 °C. Gas (O2, N2 or Ar) was continuously bubbledthrough the reaction mixture at a flow rate of 30 mL min-1.The actual power intensity of the US horn ranged from 1.2to 2.7 W cm-2, as determined by a calorimetric methoddescribed elsewhere (19). The UV lamp radiated predomi-nantly at 365 nm and the intensity and photon flux calculatedby o-nitrobenzaldehyde actinometry (20) was 4.4 × 10-8

einstein s-1 and 390 µW cm-2, respectively.Degradation Experiments. 100 mL of the aqueous dye

solution of desired concentration was taken in the jacketedcontainer. Catalyst concentration of 1 g L-1 was employedfor all the experiments as the initial rate was the same athigher loadings. Before irradiating the dye solutions with UVand/or US, the solutions were stirred in the dark for 30 minwith the catalyst to achieve adsorption-desorption equi-librium. For all the dyes, the reduction in the initialconcentration due to adsorption was not significant. pH ofthe reaction mixture was monitored during the course of thereaction and it was within 3.3-3.9. A constant UV intensitywas employed and the US intensity was varied by changingthe voltage setting in the US processor. The experimentswere carried out with an effective US power intensity of 1.2W cm-2, unless explicitly mentioned. The experiments wererepeated in triplicate and the error in measurement was lessthan 3.5%.

Sample Analysis. The aliquots at different time intervalswere centrifuged and filtered to remove the fine catalystparticles prior to analysis. The time evolution of theconcentration of the dyes was monitored by the reductionin absorbance corresponding to the characteristic absorptionwavelength (λmax) in a Shimadzu UV-1700 UV-visible spec-trophotometer. The λmax for OG, RBBR, ARS, MB and IC were482, 593, 420, 600 and 610 nm, respectively. The sampleswere quantified by a standard calibration based on Beer-Lambert’s law. The analysis by UV is adequate because theintermediates formed (which are mostly organic acids) donot absorb at λmax. Total organic carbon (TOC) levels weremonitored using Shimadzu TOC-V CSN TOC analyzer duringthe course of degradation reaction. Calibration was carriedout using potassium hydrogen phthalate for total carbon(TC) and sodium hydrogen carbonate and sodium carbonatefor inorganic carbon (IC). TOC was calculated as thedifference between TC and IC.

Dual-Pathway Network Mechanism. The proposed mech-anism of sonophotocatalytic degradation of dyes is partlybased on the previous models developed for photocatalysis(6, 18) and sonocatalysis (10, 11)

Adsorption-desorption.

TiO2 +DTTiO2 -Dads (1)

The first step in the sonophotocatalytic degradation is theestablishment of adsorption- desorption equilibrium ofthe dye (D) on the TiO2 catalyst particles, consistent with theexperiments. This is characterized by the formation ofsurface-adsorbed dye species (Dads) on TiO2. It is assumedthat these adsorbed species act as active centers where thedegradation of the substrate occurs by the attack of hydroxylradicals.

UV pathwayGeneration of charge-carriers by photolysis:

TiO2 -Dads + IUVTTiO2∗(UV)(e-, h+)-Dads (2)

The second step is the generation of charge-carriers, viz.,conduction band electrons and valence band holes by theUV excitation of TiO2 within a time scale of femtoseconds(6). The excitation occurs when the incident energy equals

or exceeds the band gap energy of the semiconductor. Thisprocess is often reversible with the recombination of electronsand holes to radiate heat, occurring in a time frame of 10-100ns (6). Herein, we take the rate of generation of charge-carriersto be dependent on the TiO2 suspension concentration andthe intensity of incident UV radiation.

Generation of hydroxyl radicals:Hole pathway:

TiO2(h+)(UV) -Dads +H2OfTiO2(OH•)ads -Dads +H+ (3)

TiO2(h+)(UV) -Dads +OH-fTiO2(OH•)ads -Dads (4)

TiO2(e-)(UV) -Dads +O2fTiO2(O2)ads -Dads (5)

Electron pathway:

TiO2(O2)ads -Dads +H+fTiO2(HOO•)ads -Dads (6)

TiO2(HOO•)ads -Dads +H+fTiO2(H2O2)ads -Dads (7)

TiO2(H2O2)ads -Dads + e-fTiO2(OH•)ads -Dads +OH-

(8)

The next step is the generation of hydroxyl radicals (OH · ),which are the precursors of degradation, by the hole andelectron pathways. In the hole pathway, TiO2 bound hydroxylspecies are generated by the oxidation of water or hydroxylanions by the holes generated by TiO2 photolysis. The electronpathway proceeds by the reduction of dissolved oxygen insolution by conduction band electrons to form superoxideradicals O2

- •

These undergo a series of proton addition reactions toform hydroperoxy radicals and hydrogen peroxide to finallyform hydroxyl radicals.

US pathwayGeneration of charge-carriers by sonolysis:

TiO2 -Dads + IUSTTiO2∗(US)(e-, h+)-Dads (9)

Hole pathway:

TiO2(h+)(US) -Dads +H2OfTiO2(OH•)ads -Dads +H+

(10)

In the parallel US pathway, excited-state TiO2 species isformed by US irradiation. This can be due to sonolumines-cence, where the light flash of average photon energy of 6

SCHEME 1. Proposed Dual-Pathway Network Mechanism forthe Sonophotocatalytic Degradation of Dyes (Dashed DottedLine, Us Pathway)

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eV is generated (21), which can lead to charge-carriergeneration or by thermolytic generation of holes in the TiO2

(22). Analogous to reaction 2, this step is dependent on TiO2

concentration and US intensity. The hydroxyl radicals arethen generated by two routes: first, the holes generated bysonolysis of TiO2 oxidize water (reaction 10), and second,hydroxyl radicals are generated by the sonolysis of water,which directly attack the TiO2 adsorbed dye to form surfacehydroxyl species (reactions 11-19).

Sonolysis of water:

H2O+ IUS98k1

OH• +H• (11)

O298k2

2O• (12)

H2O+O• 98k3

2OH• (13)

OH• +O2-• 98

k4OH-+O2 (14)

H• +O298k5

HO2 (15)

HO2 +H• 98k6

H2O2 (16)

HO2 +HO298k7

H2O2 +O2 (17)

H2O298kpyr

product (18)

Formation of hydroxyl radicals:

TiO2 -Dads +OH•fTiO2(OH•)ads -Dads (19)

Sonolysis of water, which is accompanied by a wide varietyof radical reactions, are listed by Adewuyi (10) and Colussiet al. (23). Herein, we incorporate an optimum set of reactionsand propose a simplified model for the generation of hydroxylradicals. Initially, sonolysis of water generates hydroxyl andhydrogen radicals, which are produced by the collapse ofthe oscillating cavitation bubbles in the compression phase(reaction 11). Makino et al. (24) have proved by spin-trapESR study, that these radicals are formed in the cavitationbubbles and not in the bulk liquid. The recombination of theradicals occurs in the cavity and the generation of O atomsoccurs in the gas phase, while reactions 13-18 involving thehydroperoxy radicals and H2O2 occurs in the bulk liquid only.The yield of hydroxyl radicals by this reaction is only 20% asthe rest of the radicals recombine in the cavity. O2 moleculesare also found to be thermally dissociated in hot gas bubblesto form O atoms (reaction 12) (25). These oxygen atoms reactwith water to form hydroxyl radicals (reaction 13). Since theabove two reactions are solely due to the thermal effect ofultrasound in solution, the net generation of OH · and O · isdependent on the frequency and intensity of ultrasound. Wehave also included the reaction in which the hydroxyl radicalsreact with the superoxide radicals generated by the UVpathway to form hydroxyl anions (reaction 14). Some otherside reactions include the formation of hydroperoxy radicalsand hydrogen peroxide initiated by hydrogen atom (reactions15-17). However, Nam et al. (26) have reported that the ratecoefficient for the formation of hydroperoxy radical byreaction 15 is 2.1 × 1010 M-1s-1, which is 5 orders of magnitudehigher than that of the self-reaction rate coefficient ofhydroperoxy radicals by reaction 17 (8.3 × 105 M-1s-1). Hence

the contribution of reaction 17 for the formation of H2O2 isnegligible compared to reactions 15 and 16. H2O2 thus formeddecomposes in the “hot spot” shell region to form pyrolysisproducts, represented by reaction 18. Thus, hydroxyl radicalsgenerated by the sonolysis of water react with the TiO2

adsorbed dye species and forms hydroxyl species (reaction19).

Degradation of the dye by hydroxyl radicals:

TiO2(OH•)ads -Dads +TiO2 -Dads(orD)f

intermediates(P)fCO2 +H2O (20)

Thus, the hydroxyl radicals generated by the UV and USpathways react with either the adsorbed dye or dye in bulksolution and forms complex intermediates, which furtherdegrade upon complete mineralization to CO2 and H2O.

The rate equations for reactions 11-18 in the US pathwaywere solved by assuming pseudo steady state (PSSA) on theradical species, as discussed in Appendix A (see SI 3 in theSupporting Information). It is thus clear from SI 3, eqA.6-A.10, that the concentration of O · , H · , H2O2, and OH ·generated by the US pathway are dependent only on IUS. Wenext formulate the above reactions 1-20 in the form of adual-pathway cyclic network (Scheme 1), which captures allthe essential features of the mechanism. The solutionmethodology of such a network, given by Chen and Chern(17), is based on the network reduction technique. In thistechnique, the rate of degradation of the dye is obtaineddirectly, without PSSA or the quasi-equilibrium approxima-tion, thus avoiding cumbersome calculations and restrictiveassumptions. The steps involved in the solution of the rateequation are given in Appendix B (see SI 4 in the SupportingInformation). The final expression for the concentration ofthe dye (CD) is given by

A′(CD0 -CD)+B′ ln(CD0 ⁄ CD))C′t (21)

where, A′)k01(L13+k30), B′)k30(k10+L13), C′)k01L13k30[TiO2],CD0 is the initial concentration of the dye and [TiO2] is thecatalyst concentration. Thus by fitting the experimental dataof the concentration of the dye species with eq. 21, the rateconstants, k01, k10, L13, and k30, signifying the adsorption ofthe dye onto TiO2, desorption from TiO2 surface, generationof surface hydroxyl species by UV and US pathways and thedegradation of the dye, respectively, can be determined.

Results and DiscussionThe photocatalytic, sonocatalytic and sonophotocatalyticdegradation of the dyes were carried out with CS TiO2 andDP-25. No significant degradation of the dyes was observedin the absence of the catalyst or in the absence of UV or USirradiation. Previous studies indicate that sonocatalyticactivity is greatly influenced by the presence of dissolvedgases in solution as these act as nucleation sites for cavitation.The three detrimental physical properties of the dissolvedgas are (a) specific heat ratio, (b) thermal conductivity, and(c) solubility of the gas in the solvent. As the maximumtemperature and pressure attained within a transient col-lapsing bubble are directly related to the specific heat ratio(8, 9), monatomic gases, which have a higher value of thisquantity than diatomic gases are desirable. Gases with lowthermal conductivity result in lower heat loss due todissipation, thereby leading to higher local temperatures andhence are favorable. Gases that are highly soluble in thesolvent reduce the intensity of the shock wave radiated duringthe final stages of collapse due to the cushioning effect itexerts on the cavitation bubbles (7) and hence retard thedegradation. Our investigation of the sonophotocatalyticdegradation of RBBR showed that the degradation in presenceof different gases follows the order O2 > N2 > Ar > no gas.

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This is in striking correspondence with the rate of degradationof polystyrene in toluene, reported by Price and Smith (27).It can be seen from Figure 1(a) that though the initial ratesof degradation of RBBR with CS TiO2 in presence of O2, N2

and Ar were nearly the same, the degradation was nearly90% at the end of 90 min in presence of O2, while it was only60% in presence of N2 and Ar. The observed behavior can becorrelated with the solubility of the gases in water. Ar is highlysoluble in water compared to N2 and O2 and hence it exhibitsthe slowest degradation. Between N2 and O2, though theformer is less soluble in water, it exhibits a lower activity,which can be attributed to the scavenging of hydroxyl ra-dicals by N2 to yield nitrogen and nitrous oxides. Moreover,O2 chemically participates in the electron pathway ofphotocatalysis and the formation of oxygen radicals ofsonocatalysis, thereby leading to complete degradation. But,Ar and N2 are inert gases and hence their relative contributionto the UV and US pathways is diminished. Therefore, all theother reactions were carried out with continuous bubblingof O2.

Figure 1(b) shows the effect of different operationalparameters on the degradation of 50 mg L-1 OG in O2

saturated atmosphere in presence of CS TiO2. The degradationfollows the order UV + US (1.6 W cm-2) > UV + US (1.2 Wcm-2) > UV only > US only (1.6 W cm-2). While thesonocatalytic and photocatalytic processes lead to 11 and85% degradation of OG at the end of 120 min, respectively,the sonophotocatalytic process is effective and leads tocomplete decoloration within 100 min. Thus, the synergisticeffect of US coupled to photocatalysis stems from the factthat US imparts a deaggregating effect on the catalystparticles, thereby increasing the surface area and hence thephotoactivity. Moreover, US generates extra hydroxyl radicals,which aids the photocatalytic pathway. It is also clear thatthe degradation rate is enhanced when the US intensity, i.e.,the acoustic pressure amplitude is increased. This is dueto the formation of more cavitation bubbles and theirtransient behavior in solution, which may lead to theirincrease in average size. Generally, the transient cavitationbubbles contain vapor in them, which implode violently, asthere is not much gas to cushion their collapse (8). Thiscontributes to higher local temperature and pressure andhence the generation of more hydroxyl radicals.

Figure SI 6 (see Supporting Information) shows thephotocatalytic, sonocatalytic and sonophotocatalytic deg-radation of RBBR, ARS, MB, and IC in presence of CS TiO2

and DP-25. For all the dyes, the sonocatalytic process inpresence of CS TiO2 exhibited the slowest degradation andthe sonophotocatalytic degradation in presence of CS TiO2

exhibited the fastest, and near-complete degradation. It isinteresting to note that the rate of sonophotocatalytic processwith DP-25 was the same as that of the photocatalytic processwith CS TiO2 for RBBR and significantly lower for ARS andIC. This might be due to the lower surface area and thepresence of rutile crystallites in DP-25, which may block theUV radiation incident on the dye molecule. Thus, DP-25exhibits a lesser overall activity than CS TiO2.

The kinetics of the sonophotocatalytic degradation of allthe dyes was evaluated by conducting reactions at differentinitial concentrations (50, 40, 30, 20, and 15 mg L-1) anddetermining the rate coefficients by fitting the experimentaldata with the network rate model. We have used “lsqcurvefit”,a nonlinear curve fitting module of the MATLAB optimizationtoolbox, which is based on the interior-reflective Newtonmethod. Experimental data of the evolution of the concen-tration of the dye with different initial concentrations werefit for a single value of the four rate coefficients. In each step,a subfunction was used to evaluate the concentration CD

from eq 21 by the Newton-Raphson method and the profilefed to lsqcurvefit routine. The deviation of the model fromthe experimental data is minimized by the residual definedas the sum of squares of the deviation, summed over all theexperimental data points. We have ensured that the finalvalue of the rate coefficients are essentially the global optimaby varying the rate coefficients randomly once a local solutionis attained and continuing the iteration till a smallest residualvalue is reached.

We have chosen OG as the model compound to study theeffect of US intensity on the overall degradation process.Figure 2 shows the concentration profiles of the degradationof OG with different systems. From Figure 2(a) and (b), it isevident that an increase in US intensity enhances thedegradation and complete degradation is achieved in ashorter period. Earlier studies have quantified the synergy

FIGURE 1. Concentration profiles of (a) RBBR in presence ofdifferent gases with CS TiO2 and (b) OG at different conditionswith CS TiO2.

FIGURE 2. Experimental and model predicted concentrationprofiles of OG at different conditions (solid lines, model).

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observed with the sonophotocatalytic process by a synergyparameter (10, 13), which involved the pseudo first orderrate coefficients of the individual processes. But, ourformulation of the kinetics yield the rate coefficients of theindividual steps of the sonophotocatalytic process, inde-pendent of the UV or US processes. Table 1 lists the ratecoefficients of the individual steps in the degradation process,viz., adsorption (k01), desorption (k10), formation of hydroxylradicals by the UV and US pathways (L13) and degradationof the dye by hydroxyl radical attack (k30). It is worthwhileto note that k10 increases with increase in US intensitysignifying that increasing the US power amplitude impartsa cleaning effect on the catalyst particles, thereby freeing thedye adsorbed TiO2 active site. This is also accompanied bya proportional decrease in k01. Importantly, the values of L13

and k30 are higher by 30 and 20%, respectively for the 1.6 Wcm-2 US system compared to the 1.2 W cm-2 system,suggesting that higher US intensity results in increasing thesurface area of the catalyst particles, thereby increasing thenucleation sites for bubble formation and hence morecavitation. Furthermore, charge-carrier recombination is alsoinhibited as more holes oxidize water to form hydroxylradicals (reaction 10) by the US pathway. The rate constantsfor the OG-DP-25 system also reflect the observed trends in

degradation. The low values of k01 and k10 obtained from thefitted data in Figure 2 (c) suggest that the adsorption of OGonto DP-25 was lesser owing to the lower BET surface areacompared to CS TiO2. Also, the value of k30 was nearly 35%lower than that for the CS TiO2 system, implying that thehydroxyl radical formation is lower in DP-25. This can becorrelated with the TG-DTA of the catalysts, where, CS TiO2

exhibits higher weight loss compared to DP-25.We have also quantified the sonophotocatalytic degrada-

tion of other anionic dyes, viz., RBBR, ARS, MB and IC byevaluating the rate coefficients for the CS TiO2 system. FromFigure 3, a good agreement of the model with the experi-mental concentration profile is observed for all the dyes andthe rate coefficients are listed in Table 1. More importantly,the complete degradation of ARS and IC within 60 and 30min, respectively, are well captured by k30, which is indicativeof the degradation of the dye. It is interesting to note thatthe degradation patterns of the different anionic dyesinvestigated in this study are quite different from each other.The order of degradation of the dyes (50 mg L-1) in terms ofthe time taken for complete decoloration is as follows: IC (30min) > ARS (60 min) > RBBR ≈ MB ≈ OG (105 min). Thetime taken for total degradation of IC and RBBR observed inthis study correlate well with the previous studies on

FIGURE 3. Experimental and model predicted concentration profiles of (a) RBBR, (b) ARS, (c) MB, and (d) IC with CS TiO2 (solidlines, model).

TABLE 1. Kinetic Rate Coefficients for the Different Sonophotocatalytic Systems

dye catalyst/US intensity k01 × 10-3L mg-1min-1 k10 × 10-2min-1 L13 × 10-3min-1 k30 × 10-2min-1

OG CS TiO2 + 1.2 W cm-2 0.43 0.10 0.54 0.98CS TiO2 + 1.6 W cm-2 0.38 0.16 0.79 1.21DP-25 + 1.2 W cm-2 0.22 0.12 0.47 0.65

RBBR CS TiO2 + 1.2 W cm-2 0.36 0.11 0.59 1.38ARS 1.38 8.0 5.02 1.76MB 2.78 1.35 0.72 0.41IC 3.30 2.12 2.72 2.53

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photocatalysis (28, 29). Though the operating parameters inboth the above studies are only slightly different from thisone, itseemedquitereasonableforthepurposeofcomparison.

The total time taken for the decoloration of the differentdyes can be understood by examining the structure of theindividual dyes and their degradation pathway. All the dyesinvolve the abstraction of SO4

2- anion upon irradiation withUV as the first step. IC contains intermolecular hydrogenbonds and hence the ring structure is easily cleaved duringsonophotocatalysis to form 2-amino carboxylic acid and CO2

evolution occurs by Photo-Kolbe decarboxylation reaction(28). The decomposition of ARS is found to proceed throughthe formation of organoperoxides and/or hydroxylatedintermediates to form phthalic acid as the cleaved productbefore getting completely mineralized (30). Similarly, thedegradation pathway of RBBR also involves the formation ofdicarboxylic and dihydroxy derivatives, by hydroxyl radicalreactions (29). Thus, the faster degradation of IC can beexplained based on the ease of decarboxylation of a mono-carboxylic acid, compared to that of a dicarboxylic acid inthe case of ARS and RBBR. Generally, the degradation oftriphenyl methane dyes is accompanied by the formation ofa large number of intermediates by N-demethylation andN-dealkylation reactions and the overall pathway is a complexnetwork (31-33). The products of photodegradation of acidorange 7, an azo dye, belonging to the same class of OG, havebeen thoroughly investigated (34) and the primary productsof degradation of the dye were due to the oxidative cleavagein the vicinity of the azo bond. These can partly explain theobserved trends in the sonophotocatalytic behavior amongthe different anionic dyes considered in this study.

The TOC content, which is indicative of the extent ofmineralization of the dye was monitored throughout thereaction for all the dyes. Figure SI 7 (see the SupportingInformation) elucidates the reduction in TOC level for OGwith different systems. It is evident that the trend follows thesame as that of the degradation of the dye. There was hardlyany reduction in TOC for the sonocatalytic system, while 70and 80% reduction in TOC was observed for the photocatalyticand sonophotocatalytic systems with CS TiO2, respectively.DP-25 leads to lesser mineralization of the dye and this provesthe efficacy of CS TiO2 in the conversion of organic substratesto CO2 and H2O. Furthermore, increases in US intensity wasfound to only slightly improve the rate of TOC removal inthe initial periods, though the final TOC levels were the same.Figure SI 8 (see the Supporting Information) shows the TOCprofiles for RBBR, ARS, MB, and IC for the sonophotocatalyticand photocatalytic systems with CS TiO2. It is observed thatin the initial period of 100 min, TOC concentration was thesame for both the systems, whereas at long exposure periods,the combined system exhibits synergy over the photocatalyticprocess. It is interesting to note that though all the dyes werecompletely degraded within 90 min, the TOC content isreduced only after 300 min, indicating the formation andconsumption of the intermediates in the degradation process.Hence we propose that UV plays a significant role in thedegradation of dyes in the initial period, whereas US has amore influential effect at longer exposure hours in TOCabatement. An ensemble of the above observations cor-roborate the effectiveness of the CS TiO2 catalyzed sono-photocatalytic process compared to the UV or US degradationtechniques for the decoloration and mineralization of anionicdyes.

AcknowledgmentsG.M. thanks the Department of Science and Technology forfinancial support and Swarnajayanthi fellowship.

Supporting Information AvailableSI 1: Catalyst preparation and characterization. Figure SI 2:Schematic of the experimental setup. SI 3: Appendix A

deriving the expression for the concentration of hydroxylradicals generated by the sonolysis of water. SI 4: AppendixB deriving the rate equation obtained by the dual-pathwaynetwork reduction technique. Figure SI 5: reduced dualpathway network (dashed dotted line - US pathway). FigureSI 6: Concentration profiles of (a) RBBR, (b) ARS, (c) MB, and(d) IC at different conditions. Figure SI 7: Normalized TOCprofiles for OG at different conditions. Figure SI 8: NormalizedTOC profiles for RBBR, ARS, MB, and IC at different conditionswith CS TiO2. This material is available free of charge via theInternet at http://pubs.acs.org.

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